Eta Carinae Nebula. Optical image of the Eta Carinae Nebula (NGC 3372). It is an enormous cloud of hydrogen gas (an HII region) 300 light years across, which is ionised and lit up by the ultraviolet radiation of two clusters of young, hot stars. The dark regions in the nebula are clouds of interstellar dust which absorb the light emitted by stars in these areas. The nebula is about 9000 light years away, but still appears four times as wide as the Moon in the sky. In the brightest part of the nebula lies the variable star Eta Carinae (not identifiable here). It was the second-brightest visible star during 1833-45 before fading.